This invention relates to the art of cleaning a substrate surface. In particular, it relates to the art of cleaning, without at the same time etching, a tin surface. Even more particularly, it relates to the art of cleaning tin-plated surfaces, such as tin cans which have been previously subjected to cold forming operations during which organic lubricants are applied to the tin surface as drawing aids.
Cleaning is essential as a preliminary to many surface finishing operations. Cleaning is normally required, for example, prior to corrosion preventive treatments and prior to the application of organic finishes to the surface. Cleaning is especially important in the case of metal surfaces to which organic materials have been recently applied as an aid to cold forming, which materials must then be removed in order to obtain a surface suitably receptive to an organic finish.
One example of the need for such a cleaner is in the manufacture of two piece, tin-plate, drawn and ironed cans. In the manufacture of tin-plate cans, circular blanks of tin-plated steel are first cupped and then passed through several drawing dies to iron the cup in order to form a unitary side wall and can bottom structure. These forming operations are assisted and the dies and metallic surface protected by the application of lubricants to the tin-plate surface prior to or during the forming operation. Since it is desired to have a clean surface in order to assure adhesion of a subsequently applied sanitary lacquer and/or decorative varnish, the cleaning step after forming is critical to a successful manufacturing process.
Conventional cleaners for metal surfaces are described in the Metal Finishing Guidebook and Directory, pages 195-218, (1974). Cleaners useful for tin-plate surfaces are described, for example, by R. J. Kerr in J. Soc. Chem. Inc. (London) 65:101 (1946). The Kerr article refers to alkaline aqueous composition for tin-plate cleaning containing sodium dichromate, trisodium phosphate, and sodium hydroxide.
Difficulties with tin surface cleaners have centered around an attempt to obtain a cleaner which will provide a waterbreak-free surface without unduly etching the surface. Etching results from chemical attack of the tin surface which roughens and dulls the surface. Alkaline cleaners have been found most suitable for producing the desired clean surface, but have been less than desirable because of their tendency to etch the surface. Where a smooth, shiny surface is desired such as in the case of a beverage can, etching is clearly undesirable. Furthermore, etching removes a portion of the corrosion protective tin from the surface thereby degrading the anti-corrosion qualities of the surface. It has previously been discovered that the presence of hexavalent chromium compounds in the cleaning solution will help to inhibit etching of the tin surface under the alkaline conditions employed. The use of chromium compounds is undesirable however because they are environmentally objectionable in plant effluents and because they are highly toxic and therefore of concern when used in connection with the processing of food containers such as beverage cans.
It would therefore be desirable to have available an alkaline aqueous cleaning composition which could be used to clean substrate surfaces, but which when employed to clean a tin surface will not cause undue etching.